1. Field of the Invention
The present invention relates generally to chemical vapor deposition methods and apparatus, and more particularly to a CVD reactor having an improved reactant gas injection and exhaust system for achieving more rapid and uniform material deposition.
2. Brief Description of the Prior Art
There are a large number of CVD processes that are performed inside of enclosed chambers wherein the pressure, temperature, composition of gases and other factors are controlled to produce the desired thin film deposition of various materials onto substrates such as semiconductor wafers and flat panel displays. For convenience the term wafer(s) will be used with the understanding that the following would apply to the manufacture of flat panel displays and other types of substrates.
For instance, amorphous, polycrystalline and epitaxial silicon is typically deposited onto silicon wafers by injecting silane or dichlorosilane, with or without other gases, into an enclosed vessel where the temperatures, pressure, gas flow, RF plasma intensity (when used) and wafer motion (when employed) are precisely controlled. Such processes are carried out in a wide variety of commercially available hot wall and cold wall reactors. Some of these process a single wafer at a time while others process a batch of two or more wafers at a time.
To obtain the best thickness uniformity, the operating regime of the process chamber is often chosen to be completely dominated either by the chemical reaction at the wafer surface (surface reaction rate limited) or by mass transport of the reactant to the wafer surface (mass transport limited) to make the overall deposition process least sensitive to variables which are poorly controlled in a given reactor. However, other factors such as the need for high deposition rate, high wafer capacity, or small grain size sometimes make operation near the transition region (between surface reaction-rate limited and mass transport limited) desirable. In the case of operation in either the mass transport limited regime or the transition region for a given temperature and pressure, the diffusion rate of the reactant species through the boundary layer that exists between the wafer""s surface and the bulk gas, and the relative local concentration of the desired species at the surface, have major impact upon the rate of reaction at the surface and hence upon the deposition rate.
In the prior art, methods to increase the velocity of the gas over the wafer surface have been employed to effectively decrease the thickness of the boundary layer and increase the relative concentration of the desired species at the surface for a given temperature, pressure and relative concentration of the desired species in the bulk gas.
For example, a batch reactor in the form of a conventional vertical furnace 10 is shown in FIG. 1. The furnace 10 includes a quartz envelope 12, heating coil 14, a wafer boat 16, a gas inlet 18 to a gas injection tube 20, and an exhaust 22. The tube 20 is constructed of high temperature material such as quartz, and has holes 24 (FIG. 2) along its length. The gas(es) is injected via the tube(s) 20 and directed toward the wafer boat holding wafers 26 resulting in an increased velocity of gas(es) across the wafers and a more uniform concentration of the desired reactant(s) in the bulk gas flow up and down the boat load. Although deposition rates can thereby be improved, this technique has its problems and limitations. Because the injection tube(s) 20 is contained within an isothermal chamber 28, the injection tube(s) 20 is at the same temperature as the wafers 26. Thus, unwanted deposition occurs on the tube(s) 20, especially at the injection holes 24. As the gas flow rate (velocity) is increased to achieve higher deposition rates, the localized velocity and pressure at the holes 24 can increase to the point where both excessive deposition at the holes 24 and excessive gas phase reaction within the chamber 28 occurs. The excessive gas phase reaction causes particles to be generated which can fall onto the wafers"" surfaces causing defects. Also, the excessive deposition on the injection tube 20 flakes off and these particles can also fall onto the wafers. Thus, the maximum deposition rate which yields acceptable results on the wafers is limited.
A prior art technique of rotating a wafer 30 at high speed (500 to 1500 RPM) is illustrated in a single wafer cold wall CVD reactor 32 in FIG. 3. A gas injector 34, called a showerhead directs reactant gas 36 perpendicularly toward the spinning wafer surface, thereby thinning the boundary layer for the gas flowing radially outwardly from a stagnation point at the wafer center. Again the deposition rate does increase, but this technique also has its problems and limitations. First is the problem of holding the wafer on the susceptor while rotating at such high speeds and the complexity of design for achieving such high rates of rotation in an evacuated chamber. Second is the problem of heating the wafer uniformly while allowing for the injection of the gas perpendicular to the rotating wafer""s surface which limits the maximum temperature. In addition, there is the problem of minimizing the gas turbulence to achieve the laminar type gas flow toward the wafer as required by this technique to achieve the desired uniformity. This can limit the maximum flow rate and hence the deposition rate.
In view of the above discussion, it is apparent that there is a need for an improved method and apparatus for achieving a uniform deposition rate in the process of chemical vapor deposition.
It is therefore an object of the present invention to provide an improved apparatus for chemical vapor deposition.
It is a further object of the present invention to provide an improved apparatus for applying reactant gas to a multiwafer stack.
It is a further object of the present invention to provide improved apparatus for control of the flow rate of injected and exhausted gases.
It is a still further object of the present invention to provide a heated exhaust apparatus for preventing material condensation.
It is another object of the present invention to provide removable injection and exhaust apparatus for replacement or for cleaning of material deposits.
Briefly, a preferred embodiment of the present invention includes a multiwafer chemical vapor deposition (CVD) reactor providing improved material deposition uniformity through use of improved gas injection and exhaust apparatus. The reactor includes a wafer boat for supporting a vertical stack of wafers, spaced apart for passage of a reactant gas. A preferred embodiment of the gas injector is in the form of a vertically oriented body having at a first end a gas inlet, and extending inward from a wall of the reactor towards the wafer boat, terminating in a widened injector outlet. The injector body and outlet extend vertically a distance approximating the height of the wafer boat, and the outlet is widened to provide an improved flow of gas across the wafer. A face of the injector outlet contains a plurality of gas ejecting holes, arranged to provide a uniform supply of reactant gas over each wafer surface. The exhaust manifold is similarly configured, having a plurality of exhaust ports distributed over the height of the manifold to assist in preserving the uniform flow of reactant gas across each wafer.